In general, as a switching power supply device being a flyback-type DC/DC converter, Patent Document 1, for example, discloses a first configuration that rectifies a winding voltage of an auxiliary power supply winding provided separately from a primary winding and a secondary winding of a transformer to input the winding voltage as a feedback signal to a PWM circuit. Moreover, Patent Document 1 mentioned above discloses a second configuration that insulates from the voltage rectified from the secondary winding by a photocoupler to input the voltage as the feedback signal to the pulse width modulation (PWM) circuit on the primary winding side.